Abstract: Variation across microgeographic scales presents an intriguing opportunity to explore how environmental heterogeneity shapes evolutionary processes. Evolution across small spatial scales can occur with sufficient strength of selection that exceeds gene flow, for instance with reduced levels of dispersal capacity. Further, local adaptation at this smaller scale is more likely to occur with multiple selective pressures than along a single axis. Cities, as highly fragmented and variable environments offer an excellent venue to test if small scale environmental differences influence adaptation to multiple selective pressures. While there is growing evidence of adaptation to cities, most population level comparisons have been done at coarser scales and little is known about how variation within cities alters the evolution of species. Here, we explore phenotypic variation and evolution of physiological traits within a single urban area among populations of a soil-dwelling springtail, Orchesella villosa, using a common garden design. Specifically, we use the variation in impervious surface within the urban heat island and variation of road salt application across the road network to assess intra-city springtail divergence in heat tolerance and salinity tolerance. We found an interaction between site level impervious surface and soil salinity on heat tolerance in field caught springtails with higher heat tolerance expressed from sites with higher impervious surface and soil salinity levels. Surprisingly, this interaction disappeared in the subsequent lab-reared generation suggesting that heat tolerance could be a highly plastic trait for this species. In our salinity tolerance tests of lab reared springtails, initial results show no relationship higher survival of springtails in high salt level treatments from high salt source populations suggesting divergence in this trait is not present among populations in the city. These findings suggest that environmental variation within the city is sufficient to produce phenotypic variation in some traits important for fitness and that selection for plastic phenotypes could be important in adapting to the variable urban habitat.
